A window is referred to as popup when it is relatively
small, is equipped with a short than usual title bar, is equipped with
only the System Close button on its title bar, and has thin borders. Such
a window is usually used to accompany or assist another big window or the
main window of an application. This means that a popup window is hardly
used by itself or as the main frame of an application. Based on this
description, there are some characteristics you should apply to the frame
to make it appear as a popup window.

To create a popup window, it should have the WS_POPUPWINDOW
and the WS_CAPTION styles. It should fit a relatively small
rectangle. It may also have the WS_EX_TOOLWINDOW extended style.

In this exercise, to illustrate our point, we will
create a frame-based application where a popup window is the main object.

Practical Learning: Introducing Tables

Start Microsoft Visual C++ 5, 6, or .NET

Create an Win32 Project in a folder called PopupWnd1.
Create it as an Empty Project

For this exercise, we will Use MFC In A Shared DLL. If
you don't how to do this:
in the main menu of MSVC 5 and 6, click Project -> Settings... and, in
the Microsoft Foundation Classes combo box, select Use MFC in a Shared DLL
in MSVC .NET, in the Solution Explorer, right-click the project name and
click Properties. On the right side, find Use of MFC and click it to display
its combo box. In the combo box, select Use MFC in a Shared DLL.